In a 2001 interview with the game's character designer Tetsuya Nomura published in the V-Jump magazine, he stated that he used the same approach to making character designs from Final Fantasy VIII for this game. The first thing he did was draw just the character's face, and then he discusses with the team on how "realistic" the design should be. After that he'd asked about the backstories for each character and use that to figure out the details of their designs; for example asking if they were an athletic, sportsy type, or are they are a summoner type of character. Nomura also stated that a big consideration for the game was that the world has an "Asian" vibe to it, so he tried to be conscious of that style while at the same time making sure the main characters didn't look too familiar between each other. The team asked Nomura that he only made the main protagonist Tidus stand apart somewhat from the overall world, so Tidus' design is very distinct from the Asian aesthetic.
Contributed by ProtoSnake